Dublin Institute for Advanced Studies DIAS

Dublin, Ireland

Dublin Institute for Advanced Studies DIAS

Dublin, Ireland
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Delhaye R.,Dublin Institute for Advanced Studies DIAS | Delhaye R.,National University of Ireland | Rath V.,Dublin Institute for Advanced Studies DIAS | Jones A.G.,Dublin Institute for Advanced Studies DIAS | And 2 more authors.
Solid Earth | Year: 2017

Galvanic distortions of magnetotelluric (MT) data, such as the static-shift effect, are a known problem that can lead to incorrect estimation of resistivities and erroneous modelling of geometries with resulting misinterpretation of subsurface electrical resistivity structure. A wide variety of approaches have been proposed to account for these galvanic distortions, some depending on the target area, with varying degrees of success. The natural laboratory for our study is a hydraulically permeable volume of conductive sediment at depth, the internal resistivity structure of which can be used to estimate reservoir viability for geothermal purposes; however, static-shift correction is required in order to ensure robust and precise modelling accuracy. We present here a possible method to employ frequency- domain electromagnetic data in order to correct static-shift effects, illustrated by a case study from Northern Ireland. In our survey area, airborne frequency domain electromagnetic (FDEM) data are regionally available with high spatial density. The spatial distributions of the derived static-shift corrections are analysed and applied to the uncorrected MT data prior to inversion. Two comparative inversion models are derived, one with and one without static-shift corrections, with instructive results. As expected from the one-dimensional analogy of static-shift correction, at shallow model depths, where the structure is controlled by a single local MT site, the correction of static-shift effects leads to vertical scaling of resistivity-thickness products in the model, with the corrected model showing improved correlation to existing borehole wireline resistivity data. In turn, as these vertical scalings are effectively independent of adjacent sites, lateral resistivity distributions are also affected, with up to half a decade of resistivity variation between the models estimated at depths down to 2000 m. Simple estimation of differences in bulk porosity, derived using Archie's Law, between the two models reinforces our conclusion that the suborder of magnitude resistivity contrasts induced by the correction of static shifts correspond to similar contrasts in estimated porosities, and hence, for purposes of reservoir investigation or similar cases requiring accurate absolute resistivity estimates, galvanic distortion correction, especially static-shift correction, is essential.

Botter C.,University of Stavanger | Botter C.,Dublin Institute for Advanced Studies DIAS | Cardozo N.,University of Stavanger | Qu D.,UniResearch CIPR | And 2 more authors.
Interpretation | Year: 2017

Faults play a key role in reservoirs by enhancing or restricting fluid flow. A fault zone can be divided into a fault core that accommodates most of the displacement and a surrounding damage zone. Interpretation of seismic data is a key method for studying subsurface features, but the internal structure and properties of fault zones are often at the limit of seismic resolution. In this paper, we investigate the seismic response of a vertical fault zone model in sandstone, populated with fault facies based on deformation band distributions. Deformation bands reduce the porosity of the sandstone, and condition its elastic properties. We generate synthetic seismic cubes of the fault facies model for several wave frequencies and under realistic conditions of reservoir burial and seismic acquisition. Seismic image quality and fault zone definition are highly dependent on wave frequency. At low wave frequency (e.g., 10 Hz), the fault zone is broader and no information about its fault facies distribution can be extracted. At higher wave frequencies (e.g., 30 and 60 Hz), seismic attributes, such as tensor and envelope, can be used to characterize the fault volume and its internal structure. Based on these attributes, we can subdivide the fault zone into several seismic facies from core to damage zone. Statistical analyses show a correlation between the seismic attributes and the fault internal structure, although seismic facies, due to their coarser resolution, cannot be matched to individual fault facies. The seismic facies can be used as input for reservoir models as spatial conditioning parameters for fault facies distributions inside the fault zone. However, relying only on the information provided by seismic analyses might not be enough to create high-resolution fault reservoir models. © 2017 Society of Exploration Geophysicists and American Association of Petroleum Geologists.

Cesaroni R.,National institute for astrophysics | Galli D.,National institute for astrophysics | Neri R.,Institute Of Radioastronomie Millimetrique | Walmsley C.M.,National institute for astrophysics | Walmsley C.M.,Dublin Institute for Advanced Studies DIAS
Astronomy and Astrophysics | Year: 2014

Context. The existence of disks around high-mass stars has yet to be established on a solid ground, as only few reliable candidates are known to date. The disk rotating about the ~104 L⊙ protostar IRAS 20126+4104 is probably the most convincing of these. Aims. We would like to resolve the disk structure in IRAS 20126+4104 and, if possible, investigate the relationship between the disk and the associated jet emitted along the rotation axis. Methods. We performed observations at 1.4 mm with the IRAM Plateau de Bure interferometer attaining an angular resolution of ~0".4 (~660 AU). We imaged the methyl cyanide J = 12 → 11 ground state and vibrationally excited transitions as well as the CH3 13CN isotopologue, which had proved to be disk tracers. Results. Our findings confirm the existence of a disk rotating about a ~7-10 M⊙ star in IRAS 20126+4104, with rotation velocity increasing at small radii. The dramatic improvement in sensitivity and spectral and angular resolution with respect to previous observations allows us to establish that higher excitation transitions are emitted closer to the protostar than the ground state lines, which demonstrates that the gas temperature is increasing towards the centre. We also find that the material is asymmetrically distributed in the disk and speculate on the possible origin of such a distribution. Finally, we demonstrate that the jet emitted along the disk axis is co-rotating with the disk. Conclusions. We present iron-clad evidence of the existence of a disk undergoing rotation around a B-type protostar, with rotation velocity increasing towards the centre. We also demonstrate that the disk is not axially symmetric. These results prove that B-type stars may form through disk-mediated accretion as their low-mass siblings do, but also show that the disk structure may be significantly perturbed by tidal interactions with (unseen) companions, even in a relatively poor cluster such as that associated with IRAS 20126+4104. © 2014 ESO.

Beuther H.,Max Planck Institute for Astronomy | Tackenberg J.,Max Planck Institute for Astronomy | Linz H.,Max Planck Institute for Astronomy | Henning Th.,Max Planck Institute for Astronomy | And 11 more authors.
Astrophysical Journal | Year: 2012

The ATLASGAL 870μm continuum survey conducted with the APEX telescope is the first one covering the whole inner Galactic plane (60° > l > -60° and b < 15) in submillimeter (submm) continuum emission tracing the cold dust of dense and young star-forming regions. Here, we present the overall distribution of sources within our Galactic disk. The submm continuum emission is confined to a narrow range around the Galactic plane, but shifted on average by 0.07deg below the plane. Source number counts show strong enhancements toward the Galactic center, the spiral arms, and toward prominent star-forming regions. Comparing the distribution of ATLASGAL dust continuum emission to that of young intermediate- to high-mass young stellar objects (YSOs) derived from Spitzer data, we find similarities as well as differences. In particular, the distribution of submm dust continuum emission is significantly more confined to the plane than the YSO distribution (FWHM of 0.7 and 1.1deg, corresponding to mean physical scale heights of approximately 46 and 80pc, respectively). While this difference may partly be caused by the large extinction from the dense submm cores, gradual dispersal of stellar distributions after their birth could also contribute to this effect. Compared to other tracers of Galactic structure, the ATLASGAL data are strongly confined to a narrow latitude strip around the Galactic plane. © 2012. The American Astronomical Society. All rights reserved.

Sanchez-Monge A.,National institute for astrophysics | Cesaroni R.,National institute for astrophysics | Beltran M.T.,National institute for astrophysics | Kumar M.S.N.,University of Porto | And 14 more authors.
Astronomy and Astrophysics | Year: 2013

We report on ALMA observations of continuum and molecular line emission with 0.\hbox{$\farcs$}.″4 resolution towards the high-mass star-forming region G35.20-0.74 N. Two dense cores are detected in typical hot-core tracers (e.g., CH3CN) that reveal velocity gradients. In one of these cores, the velocity field can be fitted with an almost edge-on Keplerian disk rotating about a central mass of -18 M ⊙. This finding is consistent with the results of a recent study of the CO first overtone bandhead emission at 2.3 μm towards G35.20-0.74 N. The disk radius and mass are â‰2500 au and -3 M⊙. To reconcile the observed bolometric luminosity (-3 × 104 L⊙) with the estimated stellar mass of 18 M⊙, we propose that the latter is the total mass of a binary system. © ESO, 2013.

Chira R.-A.,Max Planck Institute for Astronomy | Beuther H.,Max Planck Institute for Astronomy | Linz H.,Max Planck Institute for Astronomy | Schuller F.,European Southern Observatory | And 4 more authors.
Astronomy and Astrophysics | Year: 2013

Context. Despite increasing research in massive star formation, little is known about its earliest stages. Infrared dark clouds (IRDCs) are cold, dense and massive enough to harbour the sites of future high-mass star formation. But up to now, mainly small samples have been observed and analysed. Aims. To understand the physical conditions during the early stages of high-mass star formation, it is necessary to learn more about the physical conditions and stability in relatively unevolved IRDCs. Thus, for characterising IRDCs studies of large samples are needed. Methods. We investigate a complete sample of 218 northern hemisphere high-contrast IRDCs using the ammonia (1,1)- and (2,2)-inversion transitions. Results. We detected ammonia (1,1)-inversion transition lines in 109 of our IRDC candidates. Using the data we were able to study the physical conditions within the star-forming regions statistically. We compared them with the conditions in more evolved regions which have been observed in the same fashion as our sample sources. Our results show that IRDCs have, on average, rotation temperatures of 15 K, are turbulent (with line width FWHMs around 2 km s-1), have ammonia column densities on the order of 1014 cm-2 and molecular hydrogen column densities on the order of 1022 cm-2. Their virial masses are between 100 and a few 1000 M⊙. The comparison of bulk kinetic and potential energies indicate that the sources are close to virial equilibrium. Conclusions. IRDCs are on average cooler and less turbulent than a comparison sample of high-mass protostellar objects, and have lower ammonia column densities. Virial parameters indicate that the majority of IRDCs are currently stable, but are expected to collapse in the future. © ESO, 2013.

Vaisala M.S.,University of Helsinki | Harju J.,University of Helsinki | Harju J.,University of Turku | Mantere M.J.,University of Helsinki | And 6 more authors.
Astronomy and Astrophysics | Year: 2014

Aims. The aim of this study is to investigate the structure and kinematics of the nearby candidate first hydrostatic core Cha-MMS1. Methods. Cha-MMS1 was mapped in the NH3(1,1) line and the 1.2 cm continuum using the Australia Telescope Compact Array (ATCA). The angular resolution of the ATCA observations is 7″ (~1000 AU), and the velocity resolution is 50 m s -1. The core was also mapped with the 64m Parkes Telescope in the NH3(1,1) and (2,2) lines. Observations from Herschel Space Observatory and Spitzer Space Telescope were used to help interpretation. The ammonia spectra were analysed using Gaussian fits to the hyperfine structure. A two-layer model was applied in the central parts of the core where the ATCA spectra show signs of self-absorption. Results. A compact high column density core with a steep velocity gradient (~20 km s-1 pc-1) is detected in ammonia. We derive a high gas density (~106 cm -3) in this region, and a fractional ammonia abundance compatible with determinations towards other dense cores (~10-8). This suggests that the age of the high density core is comparable to the freeze-out timescale of ammonia in these conditions, on the order of 104 years. The direction of the velocity gradient agrees with previous single-dish observations, and the overall velocity distribution can be interpreted as rotation. The rotation axis goes through the position of a compact far-infrared source detected by Spitzer and Herschel. The specific angular momentum of the core, ~10-3km s-1 pc, is typical for protostellar envelopes. A string of 1.2 cm continuum sources is tentatively detected near the rotation axis. The ammonia spectra suggest the presence of warm embedded gas in its vicinity. An hourglass-shaped structure is seen in ammonia at the cloud's average LSR velocity, also aligned with the rotation axis. Although this structure resembles a pair of outflow lobes the ammonia spectra show no indications of shocked gas. Conclusions. The observed ammonia structure mainly delineates the inner envelope around the central source. The velocity gradient is likely to originate in the angular momentum of the contracting core, although influence of the outflow from the neighbouring young star IRS4 is possibly visible on one side of the core. The tentative continuum detection and the indications of a warm background component near the rotation axis suggest that the core contains a deeply embedded outflow which may have been missed in previous single-dish CO surveys owing to beam dilution. © 2014 ESO.

Scaife A.M.M.,Dublin Institute for Advanced Studies DIAS | Wiaux Y.,Ecole Polytechnique Federale de Lausanne | Wiaux Y.,University of Geneva
2011 30th URSI General Assembly and Scientific Symposium, URSIGASS 2011 | Year: 2011

Radio interferometry probes astrophysical signals through incomplete and noisy Fourier measurements. The optimal reconstruction of these signals is an important topic not only for current astronomical imaging but also that of the next generation of radio telescopes, for many of which image dynamic range is a key driver. The theory of compressed sensing demonstrates that incompletely sampled signals, such as those from an interferometer, may be accurately reconstructed when they are sparse or compressible in some basis. The introduction of an explicit sparsity constraint makes the method extremely versatile as it allows prior information on the signal to be introduced. Compressed sensing has been demonstrated to offer significant improvement over standard algorithms, and the flexibility of the framework and its implications for wide-field imaging are compelling, as is its potential for influencing data acquisition methods and improving data storage and transport. © 2011 IEEE.

Sanchez-Monge A.,National institute for astrophysics | Lopez-Sepulcre A.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Cesaroni R.,National institute for astrophysics | Walmsley C.M.,National institute for astrophysics | And 5 more authors.
Astronomy and Astrophysics | Year: 2013

Context. Theoretical models suggest that massive stars form via disk-mediated accretion in a similar fashion to low-mass stars. In this scenario, bipolar outflows ejected along the disk axis play a fundamental role, and their study can help characterize the different evolutionary stages involved in the formation of a high-mass star. A recent study toward massive molecular outflows has revealed a decrease in the SiO line intensity as the object evolves. Aims. The present study aims to characterize the variation of the molecular outflow properties with time and to study the SiO excitation conditions in outflows associated with high-mass young stellar objects (YSOs). Methods. We used the IRAM 30-m telescope on Pico Veleta (Spain) to map 14 high-mass star-forming regions in the SiO (2-1), SiO (5-4), and HCO+ (1-0) lines, which trace the molecular outflow emission. The FTS backend, covering a total frequency range of ~15 GHz, allowed us to simultaneously map several dense gas (e.g., N2H+, C2H, NH 2D, H13CN) and hot-core (CH3CN) tracers. We used the Hi-GAL data to improve the previous spectral energy distributions and obtained a more accurate dust envelope mass and bolometric luminosity for each source. We calculated the luminosity-to-mass ratio, which is believed to be a good indicator of the evolutionary stage of the YSO. Results. We detect SiO and HCO+ outflow emission in all fourteen sources and bipolar structures in six of them. The outflow parameters are similar to those found toward other massive YSOs with luminosities 103-104L ⊙. We find an increase in the HCO+ outflow energetics as the object evolves, and a decrease in the SiO abundance with time from 10-8 to 10-9. The SiO (5-4) to (2-1) line ratio is found to be low at the ambient gas velocity, and increases as we move to red-/blue-shifted velocities, indicating that the excitation conditions of the SiO change with the velocity of the gas. In particular, the high-velocity SiO gas component seems to arise from regions with higher densities and/or temperatures than the SiO emission at the ambient gas velocity. Conclusions. The properties of the SiO and HCO+ outflow emission suggest a scenario in which SiO is largely enhanced in the first evolutionary stages, probably owing to strong shocks produced by the protostellar jet. As the object evolves, the power of the jet would decrease and so does the SiO abundance. During this process, however, the material surrounding the protostar would have been been swept up by the jet, and the outflow activity, traced by entrained molecular material (HCO+), would increase with time. © ESO, 2013.

Odaka H.,Japan Aerospace Exploration Agency | Odaka H.,University of Tokyo | Aharonian F.,Dublin Institute for Advanced Studies DIAS | Aharonian F.,Max Planck Institute for Nuclear Physics | And 6 more authors.
Astrophysical Journal | Year: 2011

Strong iron fluorescence at 6.4keV and hard-X-ray emissions from giant molecular clouds in the Galactic center region have been interpreted as reflections of a past outburst of the Sgr A* supermassive black hole. Careful treatment of multiple interactions of photons in a complicated geometry is essential to modeling the reprocessed emissions from the dense clouds. We develop a new calculation framework of X-ray reflection from molecular clouds based on Monte Carlo simulations for accurate interpretation of high-quality observational data. By utilizing this simulation framework, we present the first calculations of morphologies and spectra of the reflected X-ray emission for several realistic models of Sgr B2, which is the most massive molecular cloud in our Galaxy. The morphology of scattered hard X-rays above 20keV is significantly different from that of iron fluorescence due to their large penetrating power into dense regions of the cloud, probing the structure of the cloud. High-resolution spectra provide quantitative evaluation of the iron line including its Compton shoulder to constrain the mass and the chemical composition of the cloud as well as the luminosity of the illuminating source. These predictions can be checked in the near future with future X-ray missions such as NuStar (hard X-rays) and ASTRO-H (both iron lines and hard X-rays). © 2011. The American Astronomical Society. All rights reserved.

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